1. Field of the Invention
The present invention relates to a beam splitting element suitable for a optical demultiplexer, a spectrophotometer, an optical measuring apparatus, or the like.
2. Description of the Related Art
Recently, a band-pass filter composed of a dielectric multilayer film and a diffraction optical element such as a diffraction grating, or a hologram are known as beam splitting elements each of which splits an optical signal, including a plurality of wavelengths, into a plurality of luminous fluxes with different wavelengths.
Such a beam splitting element has been studied for about use in various forms, for example, as a key device of a wavelength division multiplexing system. In a wavelength-division multiplex mode (WDM) of the optical communication, there is light with a plurality of different wavelengths used in a determined wavelength band. Nevertheless, when the volume of treated information becomes large, many rays of light with wavelengths used in a determined wavelength band are needed. Hence, wavelength intervals between the used wavelengths become excessively narrow in the wavelength band. Therefore, a beam splitting element that uses a diffraction optical element such as a diffraction grating is looked upon with great hope.
Here, the principle of beam splitting by a diffraction grating will be described.
It is known that it is possible to calculate an expression for finding a diffraction angle θ′ of a m-th order diffracted light, exited from the diffraction grating, from the following expression.n·sin θ−n′·sin θ′=mλ/p  (1)where, n: refraction index of incident-side medium,    n′: refraction index of outgoing-side medium,    θ: incident angle,    θ′: diffraction angle of m-th order diffracted light,    m: order number of diffraction,    λ: wavelength of incident light (incident wavelength), and    p: grating period (pitch).
As seen from Expression (1), when an incident angle θ is constant in a diffraction grating, a diffraction angle of the m-th order diffracted light is varied according to a wavelength λ of the incident light.
For example, when n=1.5, n′=1.0, θ=0°, m=1, p=100 μm, λ1=1.550 μm, and λ2=1.600 μm, a first order diffraction angle θ′ (λ1) of light with a wavelength of λ1 and the first order diffraction angle θ′ (λ2) of light with a wavelength of λ2 become θ′ (λ1)=0.8881°, and θ′ (λ2)=0.9168° respectively.
In this manner, difference Δθ′ (this is called the splitting angle) between diffraction angles of respective rays of incident light at the time when wavelength difference Δλ (=λ2−λ1) between rays of incident light is 50 nm becomes,Δθ′=θ′(λ2)−θ′(λ1)=0.0287°.
In order to perform excellent beam splitting with a diffraction grating, it is necessary to achieve a large splitting angle Δθ′, or to enlarge the physical distance between the diffraction grating and a split light-receiving section. Nevertheless, the latter method is not preferable because of drawbacks such as a large mounting package, and a complicated mechanism. Therefore, a diffraction grating with a larger splitting angle Δθ′ is requested.
On the other hand, in a wavelength-division multiplex mode (WMD) of optical communication, as described above, when the volume of treated information becomes large, it is necessary to significantly lessen (narrow) wavelength difference Δλ between a plurality of light sending different signals in a determined wavelength band. Specifically, it is not rare that the wavelength difference Δλ becomes less than 1 nm.
For example, when n=1.5, n′=1.0, θ=0°, m=1, λ1=1.550 μm, λ2=1.551 μm, that is, Δλ=0.001 μm (=1 nm), a grating period p that satisfies Δθ′=0.085° becomes p=1.69 μm from Expression (1). In order to obtain a desired splitting angle when the wavelength difference is small, a diffraction grating with an extremely small period that is about a wavelength of incident light is necessary.
In a region (λ<<p) where the grating period p is sufficiently large to the incident wavelength λ, it is well known to be able to obtain a diffraction efficiency according to a scalar diffraction theory in diffraction grating, and for example, it is mentioned in “Kaisetu-kougaku-sosi-nyuumon (Introduction to diffraction optical element”, Optoronics Co., Ltd., p.64. According to the scalar diffraction theory, a diffraction efficiency is varied according to the number of steps when a grating shape is a multilevel-stepped one, and when the numbers of step levels are 2, 4, 8, 16, and ∞, the maximum values (theoretical values) of the diffraction efficiencies are 40.5%, 81.1%, 95.0%, 98.7%, and 100% respectively.
However, in a region (λ≈p) where the grating period p is similar to the incident wavelength λ, a diffraction efficiency reduces in comparison with a diffraction efficiency calculated from the scalar diffraction theory. In general, this region is called a resonance region, and it is known that phenomena such as the reduction and polarization dependence of the diffraction efficiency happen. When the diffraction efficiency of a diffraction grating in the resonance region is found, it is possible to strictly calculate it by using a vector analysis method such as a rigorous coupled wave analysis method (RCWA).
This will be described by using concrete examples. FIG. 11 shows a conventional diffraction grating (eight-step multilevel diffraction grating), whose design values as a diffraction grating are set as grating material: SiO2 (n=1.44), outgoing-side medium: air (n′=1.00), grating period: p=1.69 μm, total grating depth: d(total)=3.01 μm, incident angle: θ=0°, and design wavelength: λ0=1.550 μm.
In such an eight-step multilevel diffraction grating, a maximum value (theoretical value) of the diffraction efficiency calculated from the scalar diffraction theory is about 95%. Nevertheless, this theoretical value does not include the loss of surface reflection (Fresnel reflection) etc.
On the other hand, FIG. 12 shows the wavelength dependence of the diffraction efficiency calculated according to RCWA. The diffraction efficiency of a plus first order diffracted light at the design wavelength λ0 (=1.550 μm) is about 47% from FIG. 12, and hence, it is understood that the diffraction efficiencies are lowered in both TE polarized light and TM polarized light (the detail will be described later though the definition of the TE polarized light and TM polarized light is shown in FIG. 2). In addition, FIG. 13 shows the diffraction efficiency in the case of a similar eight-step multilevel diffraction grating, whose design values are set as grating material: SiO2 (n=1.44), outgoing-side medium: air (n′=1.00), grating period: p=1.69 μm, total grating depth: d(total)=2.24 μm, incident angle: θ=0°, and design wavelength: λ0=1.550 μm. In this case, although the diffraction efficiency to TE polarized light is improved in comparison with the case in FIG. 11, the diffraction efficiencies to the TE polarized light and TM polarized light are largely different. In FIG. 13, the diffraction efficiencies of the plus first order diffracted light at the design wavelength (λ0=1.55 μm) are about 65% to the TE polarized light, and about 40% to the TM polarized light.
Hence, it is understood that, if a large splitting angle is achieved by reducing the grating period, the reduction and polarization dependence (difference between diffraction efficiencies to the TE polarized light and TM polarized light) of the diffraction efficiency arise. In particular, it is difficult in a two-step binary diffraction grating with periods in one direction whose number of grating steps is less than that of a multilevel diffraction grating to reduce the polarization dependence with achieving the high diffraction efficiency.
In addition, as a beam splitting element that is well known generally, there is a hologram element using the Bragg diffraction besides the above-mentioned diffraction grating. Detailed explanation concerning the hologram element is mentioned in, for example, the Bell System Technical Journal, vol.48, No.9, 1969. According to this, when the Bragg condition is nearly satisfied, it is possible to approximate the diffraction efficiency of a first order diffracted light at the design wavelength λ0 by the following expression in the case that the grating vector of the hologram element is parallel to a surface of the hologram.η(1st degree)=sin2(πΔn1d1/λ0 cos θ)  (2)where definitions are η(1st order): first order diffraction efficiency, Δn1: variance of refraction index in hologram layer, d1: hologram thickness, λ0: design wavelength, and θ: incident angle in hologram.
According to Expression (2), a condition that the first order diffraction efficiency η(1st order) becomes a maximum is to satisfy πΔn1d1/λ0 cos θ=π/2, and the diffraction efficiency theoretically becomes 100% at this time.
Nevertheless, since the periodic structure of a refraction index is achieved in a general hologram element by using material such as a photopolymer or bichromated gelatin, the refractive index difference Δn1 is near 0.02 to 0.04, and hence, a sufficient diffraction efficiency cannot be obtained unless the hologram thickness d1 is set to be large. As a result, there was a problem that, since it became a hologram element with extremely large element thickness, incident wavelength dependence and incidence angle dependence to the diffraction efficiency also became large.
In this manner, it was difficult in a predetermined wavelength band to achieve a beam splitting element, which has a high diffraction efficiency and low polarization dependence, with a conventional diffraction grating or a conventional hologram element. In particular, this was a serious problem when a large splitting angle was necessary.